There are many applications in which it is necessary to monitor fluid pressure to control a process. In medical technology, for example, pumps used to administer fluid to a patient typically include a pressure sensor for measuring the pressure of the fluid at the outlet of the pump. One such pump accepts a disposable plastic cassette that is fitted into engagement with an appropriate driver that is selectively actuated to controllably deliver medicinal fluids intravenously to a patient. The cassette comprises a plastic shell or housing made by joining a front section to a back section. A thin elastomeric sheet is encapsulated between the two sections. Fluid flows from an inlet port into a pumping chamber defined by a concave depression in one of the sections through passages formed in the housing, and a piston actuated by the pump driver displaces the elastomeric membrane to force the fluid from the pumping chamber toward an outlet port under pressure. A pressure sensor for this pump is described in U.S. Pat. No. 4,950,244, which is assigned to the same assignee as the present invention.
The pressure sensor disclosed in the above-referenced patent includes a rod that is in contact with the elastomeric membrane in the cassette at a pressure sensing location and is supported by two flexible supports that are widely spaced apart along a longitudinal axis of the rod. A strain gauge is mounted on the back of one of the flexible supports. As pressure changes in the passages of the cassette, and more particularly, at the pressure sensing location, the elastomeric membrane moves the pin axially. The movement of the pin causes a corresponding flexural displacement of the flexible supports. Flexural movement of the flexible support on which the strain gauge is mounted is detected by the strain gauge, producing a signal indicative of the pressure in the cassette at the pressure sensing location. Since the pressure sensor responds to a deflection of the flexible support indicative of fluid pressure, its response time to changes in pressure is limited by the dynamics of flexible support and movement of the elastomeric membrane.
Errors in the fluid pressure measured with the pressure sensor described above arise due to variations in the stiffness of the elastomeric membrane at varying pressures. Changes in pressure within the cassette cause substantial changes in the displacement of the elastomeric membrane. However, the stiffness of the elastomeric membrane changes as the membrane is stretched and displaced due to the fluid pressure. As a result, flexural movement of the flexible supports does not linearly track changing pressure in the cassette. These errors are not easily corrected in software, because the variation in stiffness of the elastomeric membrane as a function of pressure can differ from one cassette to another.
It will therefore be apparent that a different pressure sensor configuration is required, which is not subject to such errors. To minimize errors caused by variations in the stiffness of the elastomeric membrane, a pressure sensor is required that does not respond to flexural displacement of the elastomeric membrane. The pressure sensor must also compensate for differences in the stiffness of the elastomeric membrane in different cassettes. Further, the pressure sensor should be sufficiently compact in form so that it can monitor fluid pressures in a cassette at both proximal and distal locations relative to the pumping chamber. In addition, the pressure sensor should have a faster response time.